Head up display with an angled light pipe

ABSTRACT

A head up display includes a light pipe and a waveguide combiner. The light pipe is configured to expand a pupil in a first direction and includes an input grating and an output put grating. The light pipe also includes four elongated surfaces, and the input grating and the output grating are provided in one or more planes parallel to two of the elongated surfaces. The waveguide combiner is configured to expand the pupil in a second direction perpendicular to the first direction. The first light pipe is disposed at an angle with respect to a waveguide combiner.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/451,041, (Attorney Docket No. 17FD158P (047141-1236)) filed on Jan. 26, 2017, entitled “A HEAD UP DISPLAY (HUD) USING A LIGHT PIPE WITH ANGLED ORIENTATION WITH RESPECT TO THE COMBINER AND MICRO COLLIMATOR SYSTEM AND METHOD FOR A HEAD UP DISPLAY (HUD)”, incorporated herein by reference in its entirety, and is related to U.S. Pat. No. 9,519,089, (Attorney Docket No. 14FD004 (047141-0973)) filed on Jan. 30, 2014, entitled “HIGH PERFORMANCE VOLUME PHASE GRATINGS”, incorporated herein by reference in its entirety and assigned to the assignee of the present application, U.S. patent application Ser. No. 15/136,684 (Attorney Docket No. 47141-1108), filed on Apr. 22, 2016, entitled “A HEAD UP DISPLAY (HUD) USING A LIGHT PIPE”, incorporated herein by reference in its entirety and assigned to the assignee of the present application, which is a continuation in-part of U.S. patent application Ser. No. 14/715,332 (Attorney Docket No. 15FD011 (047141-1047), filed on May 18, 2015, entitled “A TURNING LIGHT PIPE FOR A PUPIL EXPANSION SYSTEM AND METHOD,” incorporated herein by reference in its entirety and assigned to the assignee of the present application, and U.S. patent application Ser. No. 14/814,020 (Attorney Docket No. 15FD471 (047141-1085)) filed on Jul. 30, 2015, entitled “MICRO COLLIMATOR SYSTEM AND METHOD FOR A HEAD UP DISPLAY (HUD)”, incorporated herein by reference in its entirety and assigned to the assignee of the present application, which is related to U.S. patent application Ser. No. 13/432,662 (Attorney Docket No. 11FD225 (047141-0843) filed on Mar. 28, 2012 entitled “SYSTEM FOR AND METHOD OF CATADIOPTRIC COLLIMATION IN A COMPACT HEAD UP DISPLAY (HUD),” incorporated herein by reference in its entirety and assigned to the assignee of the present application, which is a continuation-in-part application of: U.S. Pat. No. 8,634,139 (Attorney Docket No. 11FD241 (047141-0808)) filed on Sep. 30, 2011 entitled “SYSTEM FOR AND METHOD OF CATADIOPTRIC COLLIMATION IN A COMPACT HEAD UP DISPLAY (HUD),” incorporated herein by reference in its entirety and assigned to the assignee of the present application; “U.S. patent application Ser. No. 13/250,940, (Attorney Docket No. 11FD299 (047141-0819)), entitled, “HEAD UP DISPLAY (HUD) UTILIZING DIFFRACTIVE GRATINGS HAVING OPTIMIZED EFFICIENCY,” filed on Sep. 30, 2011, incorporated herein by reference in its entirety, and assigned to the assignee of the present application, now abandoned; U.S. patent application Ser. No. 13/250,858, (Attorney Docket No. 11FD126 (047141-0807)), entitled, “ULTRA-COMPACT HUD UTILIZING WAVEGUIDE PUPIL EXPANDER WITH SURFACE RELIEF GRATINGS IN HIGH REFRACTIVE INDEX MATERIALS,” filed on Sep. 30, 2011, incorporated herein by reference in its entirety, and assigned to the assignee of the present application; U.S. Pat. No. 8,903,207, (Attorney Docket No. 11FD0307 (047141-0820)), entitled, “SYSTEM FOR AND METHOD OF EXTENDING VERTICAL FIELD OF VIEW IN HEAD UP DISPLAY UTILIZING A WAVEGUIDE COMBINER,” filed on Sep. 30, 2011, incorporated herein by reference in its entirety, and assigned to the assignee of the present application; U.S. Pat. No. 8,937,772, (Attorney Docket No. 11FD096 (047141-0785)), entitled, “SYSTEM FOR AND METHOD OF STOWING HUD COMBINERS,” filed on Sep. 30, 2011 and assigned to the assignee of the present application, incorporated herein by reference in its entirety; and U.S. Pat. No. 8,749,890, (Attorney Docket No. 11FD122 (047141-0786)), entitled, “COMPACT HEAD UP DISPLAY (HUD) FOR COCKPITS WITH CONSTRAINED SPACE ENVELOPES,” filed on Sep. 30, 2011, incorporated herein by reference herein in its entirety and assigned to the assignee of the present application.

BACKGROUND

Embodiments of the inventive concepts disclosed herein relate to substrate guided displays including but not limited to head up displays (HUDs), such as, fixed HUDs and worn displays (e.g., head worn displays, helmet mounted displays, virtual glasses).

HUDs provide significant safety and operational benefits including precise energy management and conformal flight paths. These safety and operational benefits are enjoyed by operators of air transport aircraft, military aircraft, regional aircraft and high end business jets where HUDs are generally employed. These safety and operational benefits are also desirable in smaller aircraft.

Conventional HUDs are generally large, expensive and difficult to fit into smaller aircraft, such as, business and regional jets as well as general aviation airplanes. Often, conventional HUDs rely on large optical components to form adequate field of view and viewing eye box. The large optical components are often associated with collimating or non-collimating projectors and include lens, prisms, mirrors, etc. The volume of the packages including the optical components of the HUD is too large to fit within the constrained space in the cockpit of smaller aircraft. Further, conventional HUDs rely upon optical components which are generally too expensive for the cost requirements of smaller aircraft and worn displays.

Substrate guided HUDs have been proposed which use waveguide technology with diffraction gratings to preserve eye box size while reducing size of the HUD. U.S. Pat. No. 4,309,070 issued St. Leger Searle and U.S. Pat. No. 4,711,512 issued to Upatnieks disclose substrate waveguide HUDs. U.S. Pat. No. 8,634,139 discloses a catadioptric collimator for HUDs. The U.S. patent applications listed in the Cross Reference to Related Applications above disclose compact HUDS and near eye HUDs using multiple gratings, multiple waveguides, light pipes, and/or multiple waveguide layers for pupil expansion and are incorporated herein by reference in their entireties and assigned to the assignee of the present application.

SUMMARY

In one aspect, embodiments of the inventive concepts disclosed herein relate to a head up display. The head up display includes a light pipe and a waveguide combiner. The light pipe is configured to expand a pupil in a first direction and includes an input grating and an output grating. The light pipe also includes four elongated surfaces, and the input grating and the output grating are provided in one or more planes parallel to two of the elongated surfaces. The waveguide combiner is configured to expand the pupil in a second direction perpendicular to the first direction. The first light pipe is disposed at an angle with respect to a waveguide combiner.

In a further aspect, embodiments of the inventive concepts disclosed herein relate to a method of providing information to a user. The method includes providing light from a projector and providing the light from the projector to a light pipe and expanding the pupil in the first direction in the first light pipe. The method also includes providing light from the light pipe to a waveguide combiner. The light pipe has an elongated surface disposed at an angle and spaced apart from a main surface of the waveguide combiner.

In a still further aspect, embodiments of the inventive concepts disclosed herein relate to a head up display. The head up display system includes at least one light pipe, and a waveguide combiner. The at least one light pipe is spaced apart from and disposed at an angle with respect to the waveguide combiner. The light pipe is configured to expand a pupil in a first direction and provide light to an input grating on the waveguide combiner. The waveguide combiner is configured to expand the pupil in a second direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the inventive concepts disclosed herein are described with reference to the accompanying drawings, wherein like numerals denote like elements; and:

FIG. 1 is a top view schematic drawing of a head up display (HUD) display system in accordance with some exemplary embodiments of the inventive concepts disclosed herein;

FIG. 2 is a perspective view schematic drawing of the HUD system illustrated in FIG. 1;

FIG. 3 is a top view schematic drawing of a head up display (HUD) display system in accordance with some exemplary embodiments of the inventive concepts disclosed herein;

FIG. 4 is a perspective view schematic drawing of the HUD system illustrated in FIG. 3;

FIG. 5 is a top view schematic drawing of a head up display (HUD) display system in accordance with some exemplary embodiments of the inventive concepts disclosed herein;

FIG. 6 is a perspective view schematic drawing of the HUD system illustrated in FIG. 5;

FIG. 7 is a top view schematic drawing of a head up display (HUD) display system in accordance with some exemplary embodiments of the inventive concepts disclosed herein; and

FIG. 8 is a perspective view schematic drawing of the HUD system illustrated in FIG. 7.

DETAILED DESCRIPTION

Before describing embodiments of the inventive concepts disclosed herein in detail the, it should be observed that the inventive concepts disclosed herein include, but are not limited to, a novel structural combination of optical components and not in the particular detailed configurations thereof. Accordingly, the structure, methods of manufacture and use, functions, control and arrangement of components have been illustrated in the drawings by readily understandable block representations and schematic drawings, in order not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art, having the benefit of the description herein. Further, the inventive concepts disclosed herein are not limited to the particular embodiments depicted in the exemplary diagrams, but should be construed in accordance with the language in the claims.

In some embodiments, a head up display (HUD) is designed using a light pipe that does not suffer from less than desirable refractive index differences between the light pipe and the planar waveguide. In some embodiments, the light pipe is separated from the planar waveguide by an airgap and is not attached to planar waveguide with an adhesive as conventional wisdom dictates. The separation between the two optical components the HUD reduces constraints on planarity between the two optical components, thereby making the device easier to manufacture and test because the two optical components can be tested separately in some embodiments. Poor co-planarity in conventional systems can cause double images. In some embodiments, the light pipe uses a two grating design (an input grating and output grating) and does not use a turning grating or reflective array, thereby reducing drawbacks in the field of view due to very high skew ray angles in light pipe. In some embodiments, the HUD achieves a field of view (FOV) of 36 degrees (circular or square) which is greater than a conventional 25 degree circular or square FOV.

With reference to FIGS. 1 and 2, a head up display (HUD) system 100 can be utilized in various applications, including aviation, medical, naval, targeting, ground based, military, etc. The term HUD as used herein refers to a fixed HUD, a near eye display, a worn display, a helmet mounted display or any type of display using a combiner for overlaying images from an image source over a real world scene.

The HUD system 100 is configured for use in smaller cockpit environments and in worn display applications and yet provides an appropriate field of view and eye box for avionic applications in some embodiments. The HUD system 100 can be configured for use with worn components, such as, glasses, goggles, hats, helmets, etc. or be a HUD system with a fixed combiner in some embodiments. The HUD system 100 can have a variety of sizes and have a variety of display areas. A worn version of the HUD system 100 can have a display area of 40 centimeter squared or less, and a fixed version of the HUD system 100 can have a display area of more than 50 centimeters squared.

As shown in the embodiment of FIGS. 1 and 2, the HUD system 100 includes a projector 102 and a substrate waveguide system 104. The projector 102 provides light (an image) to the substrate waveguide system 104 which operates as a combiner. As shown in the embodiment of FIGS. 1 and 2, the substrate waveguide system 104 includes a light pipe 106 and a substrate combiner 108. The substrate waveguide system 104 is a transparent combiner for viewing the real world scene through main surfaces or sides 112 and 114 of the substrate combiner 108. The sides 112 and 114 are planar, flat surfaces. The substrate waveguide system 104 achieves close to a 90 degree angle between the two directions of pupil expansion and therefore provides a compact and high efficiency system with large unvignetted eye box with dispersion compensation in some embodiments. In some embodiments, the light enters the light pipe 106 as collimated light and leaves the light pipe 106 and the substrate combiner 108 as collimated light.

As shown in the embodiment of FIGS. 1 and 2, the light pipe 106 is a glass elongated rectangular prism with four elongated sides and two square or rectangular ends. The material for the light pipe 106 has a high index of refraction (e.g., greater than 1.5 in some embodiments) (e.g., 1.52). Other suitable optical materials can be used for the light pipe 106. The light pipe 106 includes an input grating 116, a beam splitter 118, and an output grating 120. The substrate combiner 108 is a glass or plastic material having a high index of refraction (e.g., greater than 1.5 in some embodiments) (e.g., 1.52). The substrate combiner 108 includes an input grating 130, a beam splitter 132, and an output grating 134.

In operation, the HUD system 100 provides images from the projector 102 via the substrate waveguide system 104 to a pilot or other operator so that the pilot or other operator simultaneously views the images and a real world scene in some embodiments. The images can include graphic and/or text information (e.g., flight path vector) related to avionic information in some embodiments. In addition, the images can include synthetic or enhanced vision images. In some embodiments, collimated light is provided to the substrate waveguide system 104 so that the pilot can view the image conformally on the real world scene through the substrate waveguide system 104.

The projector 102 includes an image source and collimating optics in some embodiments. The projector 102 provides an image from the image source and collimates the image via collimating optics for display on the substrate waveguide system 104. The projector 102 can be a collimating optical system including but not limited to any one of the collimators described in the applications incorporated herein by reference, such as, U.S. patent application Ser. No. 15/136,684, U.S. patent application Ser. No. 14/715,332, U.S. patent application Ser. No. 14/814,020, U.S. patent application Ser. No. 13/432,662, and U.S. Pat. No. 8,634,139. The projector 102 can use light emitting diode (LED) illumination, or can be a digital light projector-based (DLP-based), projector, a liquid crystal on silicon-based (LCOS-based) projector, or a laser-based projector in some embodiments. In some embodiments, the projector 102 is a monochrome projector or a color projector using a separate waveguide system 104 for each color.

As shown in the embodiment of FIGS. 1 and 2, the projector 102 and the user are disposed on respective opposing sides 112 and 114 of the substrate combiner 108. The light pipe 106 is disposed between the projector 102 and the main side 112 of the substrate combiner 108.

The input grating 116 of the light pipe 106 is disposed on an end portion 122 of faces or surfaces 142 or 144 (or surfaces parallel to the surfaces 142 and 144) of the light pipe 106 in some embodiments. As shown in the embodiment of FIGS. 1 and 2, the surfaces 142 and 144 are two of the four elongated surfaces of the light pipe 106 and are parallel to each other. The beam splitter 118 is disposed in the light pipe 106 perpendicular to the faces or surfaces 142 and 144 and between the input grating 116 and the output grating 120. The output grating 120 is disposed near an end portion 124 of the light pipe 106. The input grating 116 and the output grating 120 are matched, reciprocal gratings in some embodiments. The input grating 116 and the output grating 120 are matched in spatial frequency (e.g., have the same period) in some embodiments. The light from the projector 102 is diffracted into the light pipe 106 by the input grating 116 and propagates down the light pipe 106 by total internal reflection until it reaches the output grating 120 where it is ejected from the light pipe 106 to the input grating 130 of the substrate combiner 108.

As shown in the embodiment of FIGS. 1 and 2, the substrate combiner 108 is a rectangular prism with four elongated sides and two ends. The main sides 112 and 114 are two of the four elongated surfaces and provide a display surface. The substrate combiner 108 includes the input grating 130 disposed on a top portion 136 of main sides 112 or 114 (or a surface parallel to the sides 112 or 114) of the substrate combiner 108 in some embodiments. The beam splitter 132 is disposed in the substrate combiner 108 parallel with the sides 112 and 114 and between the input grating 130 and the output grating 134. The output grating 134 is disposed at a bottom portion 138 of the substrate combiner 108 on the sides 112 and 114 (or their parallel) in some embodiments. The input grating 130 and the output grating 134 are matched, reciprocal gratings in some embodiments. The input grating 130 and the output grating 134 are matched in spatial frequency in some embodiments. The light from the output grating 120 of the light pipe 106 is diffracted into the substrate combiner 108 by the input grating 130 and propagates down the light pipe 106 by total internal reflection until it reaches the output grating 134 where it is ejected from the substrate combiner 108 toward the user.

The input gratings 116 and 130 and the output gratings 120 and 134 can be placed on or within the local planes of the light pipe 106 and the substrate combiner 108. The input gratings 116 and 130 and the output gratings 120 and 134 can include but are not limited to volume holograms, replicated gratings or surface relief gratings. In some embodiments, the input gratings 116 and 130 and the output gratings 120 and 134 are encapsulated gratings such as those described in U.S. Pat. No. 9,519,089, incorporated herein by reference in its entirety. The input gratings 116 and 130 and the output gratings 120 and 134 are reflection type or transmission type gratings in some embodiments. In some embodiments, the output gratings 120 and 134 are rolled-K-vector output gratings. Rolled K-vector output gratings include volumetric diffraction gratings with different K vectors and the same grating period in some embodiments.

By making the gratings reciprocal, skewed rays are prevented from diffracting. The design only needs to consider the fields coming from the input couplers that are incident on the output couplers at the same angles they left the input couplers. The use of turning gratings, as used in other conventional systems, relies on the ability of the turning grating to efficiently diffract skew rays. In practice, gratings fail to perform well when the skew angle exceeds 45 degrees, especially at the higher angles >70. This puts limitations on the total FOV that can be viewed with a waveguide system containing a turning grating.

In some embodiments, an air gap or low index of refraction material is disposed between the light pipe 106 and the substrate combiner 108. The provision of the air gap provides a higher numerical aperture (NA) which results in a larger field of view. NA=square root (n² _(light pipe)−n² _(air)) where: n_(light pipe) is the index of refraction of the glass material associated with the light pipe (e.g., greater than 1.52, and equal to approximately 1.6 in some embodiments); and n_(air) is the index of refraction associated with the air gap (e.g., 1.0). If the light pipe 106 is adhered to the substrate combiner 108, the NA is decreased because the adhesive and material associated with the substrate combiner 108 have a higher index of refraction than air. The refractive index of the adhesive is greater than 1.33 in some systems. The field of view is increased by approximately 50 percent using the HUD system 100 with an air gap in some embodiments.

In addition, the air gap between the light pipe 106 and the substrate combiner 108 increases the alignment tolerances by orders of magnitude and allows for an angle to be disposed between an elongated surface (the surface 142) of the light pipe 106 and the sides 112 and 114 of the substrate combiner 108. Light pipe 106 can advantageously be built and tested separately from the substrate combiner 108 due to the separation. Further, rotating the light pipe 106 with respect to the substrate combiner 108 enables a new degree of freedom in aligning the desired field of view within the numerical aperture of the light pipe 106 which improves field of view. In some embodiments, planar waveguide combiners can be angled strategically for better fit into aircraft or other environments.

As shown in the embodiment of FIGS. 1 and 2, an angle θ between the elongated surfaces 142 and 144 of the light pipe 106 and the sides 112 and 114 is between 0 and plus or minus 45 degrees (e.g., between 5 and 25 degrees). In some embodiments, the angle between the output lens of the projector 102 and the light pipe 106 is similarly angled. In some embodiments, the angle θ1 between the output lens of the projector 102 and the light pipe 106 is a different angle than angle θ. In some embodiments, the angle between the output lens of the projector 102 and line normal to the elongated surface of the light pipe 106 is perpendicular. By rotating the light pipe 106 with respect to the substrate combiner 108 and by aligning the output grating 120 correctly, fields of view are all sent down the waveguide system 104 in one mode which eliminates multiple images in the output field. The rotation allows the field of view to be set within the desired numerical aperture of the light pipe 106 in some embodiments.

A frame or bracket can be used to secure the light pipe 106 and the substrate combiner 108 at the appropriate angle. In some embodiments, the bracket holds the light pipe 106 at its ends and the substrate combiner 108 at its top or on its sides. The bracket is plastic or metal in some embodiments.

With reference to FIGS. 3 and 4, a HUD system 100 a is similar to the HUD system 100. The HUD system 100 a includes a projector 102 a and substrate waveguide system 104 a. The user is disposed on the side 114 a of the substrate combiner 108 a in some embodiments. A light pipe 106 a is disposed in front of (e.g., the user's side of) the main side 114 a of the substrate combiner 108 a. The projector 102 a and the substrate combiner 108 a are disposed behind the side 112 a in some embodiments.

With reference to FIGS. 5 and 6, a HUD system 100 b is similar to the HUD system 100. The HUD system 100 b includes a projector 102 b and substrate waveguide system 104 b. The user is disposed on the side 112 b of substrate combiner 108 b in some embodiments. A light pipe 106 b is disposed between the main side 114 b of the substrate combiner 108 b and the projector 102 b. The elongated side 144 b faces the main side 114 b, and the elongated side 142 b faces the output of the projector 102 b.

With reference to FIGS. 7 and 8, a HUD system 100 c is similar to the HUD system 100. The HUD system 100 c includes a projector 102 c and a substrate waveguide system 104 c. The user is disposed on the main side 114 c of substrate combiner 108 c in some embodiments. A light pipe 106 c is disposed behind the main side 112 c of the substrate combiner 108 c, and the projector 102 c is disposed in front of the elongated surface 142 c of the light pipe 106 c.

The HUD systems 100 and 100 a-c can be rotated at any angle to provide different orientations (upside down, rotate 90 degrees, 270 degrees). The same reference numerals with different suffixes a-c in FIGS. 1-8 are intended to show components that have identical or similar structure and functionality. In some embodiments, the projector 102 is one of the projectors 30, 500, 700 and 750 described in Exhibit B of the provisional application incorporated herein by reference in its entireties. In some embodiments, the projector 102 is configured to provide an exit pupil between 3 mm and 5 mm in diameter and has a cubic beam splitter with a physical size of 4.5 mm to 15 mm per side for HWDs. In some embodiments, the projector 30 is configured to provide an exit pupil between 2 mm and 25 mm in diameter.

It is understood that while the detailed drawings, specific examples, material types, thicknesses, dimensions, and particular values given provide exemplary embodiments of the inventive concepts disclosed herein, the exemplary embodiments are for the purpose of illustration only. The inventive concepts disclosed herein are not limited to the precise details and conditions disclosed. For example, although specific types of optical component, shapes, dimensions and angles are mentioned, other components, dimensions and angles can be utilized. Various changes may be made to the details disclosed without departing from the spirit of the inventive concepts disclosed herein which are defined by the following claims. 

What is claimed is:
 1. A head up display, comprising: a light pipe configured to expand a pupil in a first direction and comprising an input grating and an output put grating, the light pipe comprising four elongated surfaces comprising at least two elongated surfaces that are parallel to each other, wherein the input grating and the output grating are provided in one or more planes parallel to the two parallel elongated surfaces or on at least one of the elongated surfaces; and a waveguide combiner in optical communication with the light pipe and comprising a main surface disposed at an angle with respect to the two parallel elongated surfaces, the waveguide combiner being configured to expand the pupil in a second direction along the main surface perpendicular to the first direction.
 2. The head up display of claim 1, wherein the angle is greater than 0 degrees and less than 45 degrees, and wherein the waveguide combiner is spaced apart from the first light pipe.
 3. The head up display of claim 2, wherein the input grating and the output grating are matched.
 4. The head up display of claim 3, wherein the input grating and the output grating are reciprocal.
 5. The head up display of claim 1, wherein the angle is more than 5 degrees and less than 25 degrees.
 6. The head up display of claim 1, wherein the waveguide combiner comprises an input grating and an output grating, wherein the input grating and the output grating are provided in one or more planes parallel to the main surface or on the main surface.
 7. The head up display of claim 1, further comprising: a collimator disposed in front of the input grating of the light pipe.
 8. The head up display of claim 7, wherein the light pipe and the waveguide are part of a head worn display.
 9. The head up display of claim 8, wherein the light pipe comprises a first beam splitter disposed between the input and output gratings of the light pipe, the first beam splitter being disposed in a plane perpendicular to the two elongated surfaces, and wherein the waveguide combiner comprises two main surfaces, an input grating, an output grating, and a second beam splitter disposed between the input and output gratings of the waveguide combiner, and wherein the input and output gratings of the waveguide combiner are disposed in a plane parallel to the main surface.
 10. A method of providing information to a user, the method comprising: providing light from a projector; providing the light from the projector to a light pipe and expanding the pupil in the first direction in the first light pipe; and providing the light from the light pipe to a waveguide combiner having a main surface, wherein the light pipe has an elongated surface disposed at an angle and spaced apart from the main surface of the waveguide combiner.
 11. The method of claim 10, wherein the light pipe comprises an input grating and an output grating associated with the elongated surface.
 12. The method of claim 11, wherein the input grating and the output grating are matched reciprocal gratings.
 13. The method of claim 10, further comprising: diffracting light out of the wave guide and expanding the pupil in a second direction perpendicular to the first direction.
 14. A head up display system, comprising: at least one light pipe having four elongated surfaces, wherein a first pair of the elongated surfaces are parallel to each other and a second pair of the elongated surfaces are perpendicular to the first pair of the elongated surfaces; and a waveguide combiner having a main surface for viewing an image and an input grating, the waveguide being disposed such that the first pair of the elongated surfaces are spaced apart from and disposed at an angle with respect to the main surface of the waveguide combiner, wherein the at least one light pipe is configured to expand a pupil in a first direction and provide light to the input grating of the waveguide combiner, the waveguide combiner being configured to expand the pupil in a second direction on the main surface.
 15. The head up display system of claim 14, wherein the at least one light pipe comprises a volumetric input grating and a volumetric output grating.
 16. The head up display system of claim 14, wherein the output grating is a reciprocal grating to the input grating.
 17. The head up display system of claim 14 further comprising a collimator.
 18. The head up display system of claim 15, wherein the waveguide combiner comprises a beam splitter disposed between the input grating on the waveguide combiner and an output grating on the waveguide combiner and wherein the light pipe comprises a beam splitter disposed between the volumetric input grating and the volumetric output grating.
 19. The head up display system of claim 14, wherein the head up display is a fixed head up display or a head worn display.
 20. The head up display system of claim 15, wherein the volumetric output grating is a rolled k vector output grating. 